Recently, fiber optic communication networks called FTTx (Fiber To The x) such as a fiber optic communication network reaching a user's home are widespread. In such a fiber optic communication network, an optical amplifying apparatus is used for the purpose of compensating for transmission losses in a transmission path and distribution losses in a distributor that distributes optical signals between a plurality of subscribers.
Such an optical amplifying apparatus may be, for example, a known fiber-type optical amplifying apparatus (EDFA: Erbium Doped Fiber Amplifier) that amplifies an optical signal by inputting an optical signal such as a video signal and also inputting a pump light from an optical pump source into an optical fiber having a core portion doped with erbium, which serves as an optical amplification substance. Further, in recent years, it is known to dope the core portion with ytterbium that enables a high-power laser having a watt-class output as an absorption band to be used as an optical pump source. It is also known to use a double-clad optical fiber in which, in order to increase a pump light intensity that can be coupled in a core portion, an optical signal is propagated in a single mode through a core portion and a pump light from a high-power multimode laser light source is propagated in a multimode through a cladding portion surrounding the core portion (see Japanese Laid-Open Patent Publication No. 2008-53294).
An optical amplifying apparatus utilizing an optical fiber doped with erbium and ytterbium is often used for the purpose of amplifying about one or two waves of an optical signal in a 1550-1560 nm band that has a high conversion efficiency in the optical fiber. FIG. 14 is a graph showing amplification characteristics for an output of such an optical amplifying apparatus split into sixteen branches. In FIG. 14, the horizontal axis indicates a wavelength of an optical signal and the vertical axis shows an optical power. This example is an optical spectrum obtained when a signal of 1550 nm was amplified.
The aforementioned optical amplifying apparatus utilizing an optical fiber doped with erbium and ytterbium has been employed in an FTTx system in which signals having a wavelength band of generally 1550-1560 nm are widely used. However, an amplification bandwidth is about 25 nm at the broadest. Therefore, there is a drawback that a bandwidth is rather narrow for collectively amplifying optical signals used in communications that are wavelength division multiplexed (WDM: Wavelength Division Multiplex) and in an entire C-Band (1530-1560 nm).
Accordingly, it is an object of the invention to provide an optical amplifying apparatus that can collectively amplify wavelength-division multiplexed optical signal in a broader band than in the related art.